The human arm as a redundant manipulator: the control of path and joint angles

Cruse H, Brüwer M. The human arm as a redundant manipulator: the control of path and joint angles. Biological cybernetics. 1987;57(1-2):137-144.The movements studied involved moving the tip of a pointer attached to the hand from a given starting point to a given end point in a horizontal plane. Three joints — the shoulder, elbow and wrist —were free to move. Thus the system represented a redundant manipulator. The coordination of the movements of the three joints was recorded and analyzed. The study concerned how the joints are controlled during a movement. The results are used to evaluate several current hypotheses for motor control. Basically, the incremental changes are calculated so as to move the tip of the manipulator along a straight line in the workspace. The values of the individual joints seem to be determined as follows. Starting from the initial values the incremental changes in the three joint angles represent a compromise between two criteria: 1) the amount of the angular change should be about the same in the three joints, and 2) the angular changes should minimize the total cost of the arm position as determined by cost functions defined for each joint as a function of angle. By itself, this mechanism would produce strongly curved trajectories in joint space which could include additional acceleration and deceleration in a joint. These are reduced by the influence of a third criterion which fits with the mass-spring hypothesis. Thus the path is calculated as a compromise between a straight line in workspace and a straight line in joint space. The latter can produce curved paths in the workspace such as were actually found in the experiments. A model calculation shows that these hypotheses can qualitatively describe the experimental findings

A Single-Rate Context-Dependent Learning Process Underlies Rapid Adaptation to Familiar Object Dynamics

Motor learning has been extensively studied using dynamic (force-field) perturbations. These induce movement errors that result in adaptive changes to the motor commands. Several state-space models have been developed to explain how trial-by-trial errors drive the progressive adaptation observed in such studies. These models have been applied to adaptation involving novel dynamics, which typically occurs over tens to hundreds of trials, and which appears to be mediated by a dual-rate adaptation process. In contrast, when manipulating objects with familiar dynamics, subjects adapt rapidly within a few trials. Here, we apply state-space models to familiar dynamics, asking whether adaptation is mediated by a single-rate or dual-rate process. Previously, we reported a task in which subjects rotate an object with known dynamics. By presenting the object at different visual orientations, adaptation was shown to be context-specific, with limited generalization to novel orientations. Here we show that a multiple-context state-space model, with a generalization function tuned to visual object orientation, can reproduce the time-course of adaptation and de-adaptation as well as the observed context-dependent behavior. In contrast to the dual-rate process associated with novel dynamics, we show that a single-rate process mediates adaptation to familiar object dynamics. The model predicts that during exposure to the object across multiple orientations, there will be a degree of independence for adaptation and de-adaptation within each context, and that the states associated with all contexts will slowly de-adapt during exposure in one particular context. We confirm these predictions in two new experiments. Results of the current study thus highlight similarities and differences in the processes engaged during exposure to novel versus familiar dynamics. In both cases, adaptation is mediated by multiple context-specific representations. In the case of familiar object dynamics, however, the representations can be engaged based on visual context, and are updated by a single-rate process

The Inactivation Principle: Mathematical Solutions Minimizing the Absolute Work and Biological Implications for the Planning of Arm Movements

An important question in the literature focusing on motor control is to determine which laws drive biological limb movements. This question has prompted numerous investigations analyzing arm movements in both humans and monkeys. Many theories assume that among all possible movements the one actually performed satisfies an optimality criterion. In the framework of optimal control theory, a first approach is to choose a cost function and test whether the proposed model fits with experimental data. A second approach (generally considered as the more difficult) is to infer the cost function from behavioral data. The cost proposed here includes a term called the absolute work of forces, reflecting the mechanical energy expenditure. Contrary to most investigations studying optimality principles of arm movements, this model has the particularity of using a cost function that is not smooth. First, a mathematical theory related to both direct and inverse optimal control approaches is presented. The first theoretical result is the Inactivation Principle, according to which minimizing a term similar to the absolute work implies simultaneous inactivation of agonistic and antagonistic muscles acting on a single joint, near the time of peak velocity. The second theoretical result is that, conversely, the presence of non-smoothness in the cost function is a necessary condition for the existence of such inactivation. Second, during an experimental study, participants were asked to perform fast vertical arm movements with one, two, and three degrees of freedom. Observed trajectories, velocity profiles, and final postures were accurately simulated by the model. In accordance, electromyographic signals showed brief simultaneous inactivation of opposing muscles during movements. Thus, assuming that human movements are optimal with respect to a certain integral cost, the minimization of an absolute-work-like cost is supported by experimental observations. Such types of optimality criteria may be applied to a large range of biological movements

Same game, different rules? Gender differences in political participation

We investigate gender gaps in political participation with 2004 ISSP data for 18 advanced Western democracies (N: 20,359) using linear and logistic regression models. Controlling for socio-economic characteristics and political attitudes reveals that women are more likely than men to have voted and engaged in ‘private’ activism, while men are more likely to have engaged in direct contact, collective types of actions and be (more active) members of political parties. Our analysis indicates that demographic and attitudinal characteristics influence participation differently among men and among women, as well as across types of participation. These results highlight the need to move toward a view of women engaging in differing types of participation and based on different characteristics.

Feature Weighting for Lazy Learning Algorithms

: Learning algorithms differ in the degree to which they process their inputs prior to their use in performance tasks. Many algorithms eagerly compile input samples and use only the compilations to make decisions. Others are lazy: they perform less precompilation and use the input samples to guide decision making. The performance of many lazy learners significantly degrades when samples are defined by features containing little or misleading information. Distinguishing feature relevance is a critical issue for these algorithms, and many solutions have been developed that assign weights to features. This chapter introduces a categorization framework for feature weighting approaches used in lazy similarity learners and briefly surveys some examples in each category. 1.1 INTRODUCTION Lazy learning algorithms are machine learning algorithms (Mitchell, 1997) that are welcome members of procrastinators anonymous. Purely lazy learners typically display the following characteristics (Aha, 19..

Ett liv i förändring Att leva med hiv/aids i västvärlden

År 1981 kom den första rapporten om en tidigare okänd sjukdom som senare kom att benämnas hiv. När hiv- viruset upptäcktes orsakade det stor oro, mycket på grund av att människor inte riktigt visste vad de hade att göra med. Trots ökad kunskap och medicinska framsteg fortgår spridningen av denna allvarliga sjukdom. I detta arbete beskrivs virusets uppbyggnad samt aids begreppet, men främst ligger fokus på hivpositivas och deras anhörigas upplevelser av sjukdomen. Drabbade och deras närstående är på olika sätt föremål för det sociala stigma som sjukdomen medför. De som har tillgång till bromsmediciner kan idag vänta sig att leva ett långt liv trots hiv- viruset, något som aktualiserar behovet av en ökad förståelse från omgivningen om hur sjukdomen påverkar livet. Syfte med studien är att beskriva hur det är att leva med hiv/aids i västvärlden idag, utifrån ett patient- och anhörigperspektiv. Arbetet är utformat som en litteraturstudie med kvalitativ forskningsansats. Resultatet är indelat i 3 huvudteman. Första temat är ett liv i förändring. Där beskrivs hur diagnosen kan mottas och vilka konsekvenser hiv- statusen kan få för levnadsförhållandena. Det andra temat är att mötas av okunskap, här tas det upp hur kunskap om bland annat smittspridningen är viktig för ett gott bemötande. Tredje temat är att leva med en stämpel, där beskrivs hur yttre och inre stigmatisering orsakar hinder och smärta, och hur den är en betydande faktor när det kommer till om man ska vara öppen med sin sjukdom eller ej. I diskussionen tas det upp hur öppenhet och tolerans kan främja en bättre levnadssituation för de personer som är smittade med hiv.Program: Sjuksköterskeutbildnin